[graphql] fix backfill target keys (#25964)

in #25736 I mistakenly thought
filtering down to the nodes in the target set in the first pass would
act as the previous `if k in self.target_subset.asset_keys` but the list
becomes filled with `parent_keys` so need to preserve that check

## How I Tested These Changes

updated test so it failed without this fix 

## Changelog

[ui] no longer display extraneous assets in the asset backfill page, a
problem introduced in 1.9.2